Hypertrophic cardiomyocyte growth is regulated by input from G-protein coupled receptors (GPCRs) and by pathways initiated by biomechanical stretch. Proper integration of these pathways may be necessary to prevent the transition from physiological hypertrophy to heart failure. This project focuses on a GPCR, a cytoskeletal protein and a calcium regulated kinase, which may mediate communication between extracellular and intracellular signals and are poorly explored but plausible regulators of cardiomyocyte growth. Specific Aim #1 examines the involvement of the LPA and LPB family of GPCRs in cardiomyocyte signaling. The specific LP receptor subtypes expressed in mouse and rat cardiomyocytes, and the signaling pathways utilized by these receptors will be defined, in vivo responses to their ligands, (LPA and S1P) determined, and the requirement for the LP receptors in responses to pressure overload (TAC) and infarction will be analyzed in receptor knockout mice. Specific Aim #2 examines the basis for the requirement for the LIM domain cytoskeletal protein FHL1 in responses to TAC. Experiments will examine altered G-protein and kinase signaling pathways in FHL1 knockout mice, and identify potential FHL1 interacting proteins in a mouse heart library by yeast 2-hybrid and by examining co-expression, co-localization and co-immunoprecipitation of FHL1 and its partners. Specific Aim #3 tests the hypothesis that the focal adhesion/Z-line/cytoskeletal proteins integrate signals from GPCRs and stretch. Proposed studies examine the ability of GPCR responsiveness, and changes in components of this pathway in models of hypertrophy and failure. Specific Aim #4 examines the activation of, and signaling role for CaMKII-delta, in hypertrophic growth and development of heart failure. Proposed studies will examine the activation of CaMKII, compare phenotypes in transgenic mice expressing nuclear (delta-B) or cytoplasmic (delta-C) CaMKII isoforms, examine effects of the delta-B and delta-C isoforms on protein phosphorylation and Ca2+ handling, and use CaMKII-delta knockout mice to test the requirement for CaMKII in Ca2+ handling, hypertrophy and failure.